DE-OS 23 28 853 discloses hollow cellulose fibers in which the cellulose has been regenerated from cuoxam solutions. This reference assumes that structures of previously known conventional cellulose (flat) membranes made of copper oxide-ammonia-regenerated cellulose have finely distributed dialysis pores or gaps with transverse measurements on the order of 10 to 90 A (A =angstrom), approximately 30 A on the average.
It is known that cellulose is composed of crystalline and amorphous parts. When the fine gap structure of a cellulose material was compared with the concept of "pores" of a thin cellulose film, it was assumed that the crystalline part plays almost no role in the dialysis of low-molecular-weight substances, while the amorphous part actually contributes to the dialysis action. During dialysis, the amorphous parts of the cellulose swell as water molecules penetrate between the cellulose molecules. As a result, enlarged gaps appear in the molecular chains through which the low-molecular-weight substances can pass during dialysis.
It has also been assumed that hollow fibers obtained by deacetylation of cellulose triacetate also consist of cellulose and that the "pore" dimensions and the nature of the distribution of the "pores" correspond to the properties of a cellophane membrane.
Regarding the hollow cellulose fibers produced in accordance with DE-OS 23 28 853, the gap passageways in the known hollow fibers are described as having transverse measurements of 200 A at most and it is assumed, on the basis of these figures, that the average value for the size of the fine gaps has been significantly increased, so that dialysis performance was improved relative to the values previously attainable.
These known hollow fibers have been viewed under an electron microscope with a magnification of 20,000.times.. Fine passageways in the form of a reticulate structure with a maximum 200 A passage width have been observed not only in cross sections but also in lengthwise sections of the hollow fibers. These passageways appear to be uniformly distributed throughout the entire fiber structure and form a network of fibrils. These known hollow fibers were produced by extruding a spinning fluid of copper ammonium cellulose through an external annular spinneret and simultaneously extruding a strand of core fluid through an internal central opening located concentrically with respect to the outer annular spinneret opening. The strand consists of an uncoagulated (relative to the spinning fluid) organic liquid solvent so that a liquid thread composed of a jacket and a core is formed and drops freely under the influence of gravity through a gaseous area approximately 300 mm long to stretch the fiber and reduce the total diameter, whereupon the stretched liquid thread is guided into a coagulation area and brought into contact with an aqueous NaOH solution.
An example of the process includes a spinning fluid containing 10 wt.% cellulose, 3.6 wt.% copper, and 7 wt.% ammonium and having a viscosity of 2,000 poise (corresponding to 200 Pas).
A dialysis membrane made of regenerated cellulose is known from DE 28 23 985-Al, said membrane being distinguished by an ultrafiltration capacity of 7 to 30 ml/h m2 mm Hg (corresponding to 14 to 60 pm.multidot.s.sup.-1 .multidot.Pa.sup.-1) and an average molecular permeability of 4.times.10.sup.-3 to 12.times.10.sup.-3 cm/min (corresponding to 0.65 to 2.0.mu..multidot.s.sup.-1) and by the fact that it is regenerated from cuoxam solutions.
In this known dialysis membrane, the ultrafiltration rate is not maintained under ordinary storage conditions, but decreases in a clearly measurable fashion.
In conjunction with the sterilization of cellulose membranes of this kind using moist heat treatment at 105.degree. to 148.degree. C., described in DE-PS 28 11 551, depending on the type of cellulose, permeability relative to water is significantly reduced. After autoclave treatment using steam alone, the UFR rate of the membrane decreases so drastically that the membrane becomes worthless. Filling the membrane with physiological saline solution is reported to prevent this decrease.
DE-PS 28 11 551 also refers to a publication of Enka Glanzstoff AG which deals with copper ammonium cellulose tubes and hollow fibers, in which the importance of treatment and storage of such membranes at 23.degree. C. and 50% relative humidity is mentioned. In this connection, a warning is given against moistening and redrying because the outstanding permeability properties are then reduced.
German Patent 29 06 576 discloses a method of spinning hollow fibers from regenerated cellulose for semipermeable membranes by extruding a cellulose-cuoxam solution through the annular slot of a hollow fiber spinneret in aqueous caustic soda and a cavity-forming liquid through the inner bore o the hollow fiber spinneret, followed by a conventional aftertreatment. The method is characterized by the fact that the hollow fiber spinneret is submerged in the aqueous caustic soda and the ratio of the pulloff rate of the hollow fiber at the first pulloff roller to the extrusion rate of the cellulose-cuoxam solution from the annular slot of the hollow fiber spinneret is 1.00 to 1.05 and the direction of thread travel from the hollow fiber spinneret to the first pulloff roller forms an acute angle with the axis of the hollow fiber spinneret opening.
The goal of this known method is to provide a method of spinning hollow fibers from regenerated cellulose for semipermeable membranes, making it possible to generate hollow fibers with exact cross-sectional shapes predetermined by the shape of the spinnerets. The fibers manufactured in this way are also characterized by their outstanding properties such as semipermeable membranes and a mechanical strength which is not adversely affected. In particular, hollow fiber shapes with an eccentric cross section can be produced in an outstanding fashion using this known method. An arrangement with the hollow fiber spinneret in the bottom of the coagulation tank is assigned less significance which owes to technical problems in sealing and spinning.
A method of producing a hollow fiber from regenerated copper ammonium cellulose with an axially located cylindrical bore is known from European Patent 0 076 442, said bore extending over the length of the fiber and being filled with gas in the following steps:
1) Extruding a spinning solution made of copper ammonium cellulose through an annular opening, whereby a fiber extrudate is formed with a hole, while gas is blown simultaneously into the fiber extrudate into the bore of the fiber through a blowing tube located in the center of the annular opening; PA1 2) Dropping the fiber in free fall in an air space and allowing it to submerge essentially perpendicularly to a depth of 2 to 20 mm, preferably up to a depth of 30 mm, below the surface of a coagulation bath, solely under the influence of the downward force during free fall; PA1 3) Guiding the fiber extrudate through the coagulation bath, whereby a hollow fiber is obtained with an axially disposed cylindrical bore; and
4) Refining and drying the hollow fibers thus produced.
The long air gap required for this purpose necessitates spinning solutions of high viscosity. In addition, the requirement that the fiber in free fall submerges itself up to 20 mm deep in the coagulation bath poses considerable difficulties.
Comprehensive studies of known hollow dialysis fibers of copper ammonium cellulose have shown that the adverse affects on the UFR are clearly dependent on the storage conditions. Thus, it has been found that when such hollow fibers are stored at relatively high humidity, the UFR decreases to about 20% of the original value in less than ten days.
When stored in the swollen (wet) state, there is no decrease in the UFR. After redrying of such membranes, however, the UFR falls to values similar to those seen for storage at 100% humidity. Hence, storage in the wet state is not practical because the storage time must remain limited in order to prevent the risk of microbiological and chemical decomposition processes in particular.